The drive for environmentally friendly vehicles has led to multiple innovations in the recent past. One of the major areas of research which has led to the viability of electric vehicles is in power systems and, more specifically, in the power systems used to charge the batteries in these electric or sometimes hybrid vehicles. Such power systems generally include DC/DC converters which convert DC power from one voltage into another, more useful, voltage.
Full-bridge topology is the most popular topology used in the power range of a few kilowatts (1-5 KW) for DC/DC converters. Since the switch ratings are optimized for the full-bridge topology, this topology is extensively used in industrial applications. High efficiency, high power density and high reliability are the prominent features of this topology.
In the range of a few kilowatts, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are mostly used to implement the full-bridge converters. In order to have robust and reliable operation, MOSFETs should be switched under zero voltage. Operating at Zero Voltage Switching (ZVS) decreases the converter switching losses and provides a noise free environment for the control circuit. Zero voltage switching is usually achieved by providing an inductive current flowing out of the full-bridge legs during the switch turn-on and by placing a snubber capacitor across each switch during the switch turn-off. In a practical full-bridge configuration, the internal drain-to-source capacitance of the MOSFET is utilized as the snubber capacitor, the series inductor is usually the leakage inductance of the power transformer, and the parallel inductor is implemented by using the magnetizing inductance of the power transformer. Thus, external passive components are not required and this makes the power circuit very simple and efficient. However, the full-bridge converter with the series inductor loses its ZVS capability at light loads, and the converter with the parallel inductor loses its ZVS under heavy loads. Loss of ZVS means extremely high switching losses at high switching frequencies and very high EMI due to the high di/dt of the snubber discharge current. Loss of ZVS can also cause a very noisy control circuit, which leads to shoot-through and loss of the semiconductor switches. The ZVS range can be extended by increasing the series inductance. However, having a large series inductance limits the power transfer capability of the converter and reduces the effective duty ratio of the converter.
In battery charger applications, ZVS is important since the converter might be operating at no-load for a long period of time. As an example, when the battery is charged, the load is zero and the converter should be able to safely operate under the zero load condition. Since ZVS in conventional full-bridge PWM converters is achieved by utilizing the energy stored in the leakage inductance to discharge the output capacitance of the MOSFETs, the range of the ZVS operation is highly dependent on the load and on the transformer leakage inductance. Thus, conventional full-bridge converters are not able to ensure ZVS operation at light loads.
Resonant topologies can provide soft-switching. However, in order to guarantee ZVS in resonant converters, a high value of reactive current circulation is required especially for a wide range of load variations. This leads to a bulky resonant tank, lower power density, and lower efficiency.
Auxiliary commutated ZVS full bridge converter topologies suitable for low power applications have been reported in the literature. In these converters, an auxiliary circuit is used to produce the reactive current for the full-bridge switches. The auxiliary circuit is working independent of the system operating conditions and is able to guarantee zero voltage switching from no-load to full-load. Although this topology seems very suitable for the battery charger application, there are some setbacks related to the auxiliary circuit. Since the auxiliary circuit should provide enough reactive power to guarantee ZVS at all operating conditions, the peak value of the current flowing through the auxiliary inductor is very high, thereby drastically increasing the MOSFET conduction losses. Also, due to the fact that the voltage and frequency across the auxiliary inductor is very high, the core losses of this inductor are also high. In addition, too much reactive current leads to large voltage spikes on the semiconductor switches due to the delay in the body diode turn-on.
There is therefore a need for methods, circuits, and devices which can mitigate if not overcome the shortcomings of the prior art. From the above, the lowering if not the elimination of the voltage spikes while providing for ZVS conditions would be desirable.